The present invention relates to voltage transformers comprising multilayer structures of piezoelectric ceramics, so-called piezoelectric transformers. Furthermore, the present invention relates to switch mode power supplies that contain such a piezoelectric transformer as part of a piezoelectric converter.
Electromagnetic transformers based on wound coils were frequently used in the past to generate high voltage as well as to generate low voltage. Such electromagnetic transformers comprise an electrical conductor that is wound onto a core of a ferromagnetic material. Since a large number of windings is required for a large voltage transfer ratio, electromagnetic transformers with sufficient effectiveness can usually only be produced with difficulty in miniaturized form. Moreover, an electromagnetic transformer has many disadvantages in high-frequency applications, in particular on account of the magnetic material, such as, for example, rise of the hysteresis loss, vortex current losses or skin effect losses in the conductor. These losses limit the practically realizable frequency range of magnetic transformers to about 500 kHz.
In order to overcome these and other problems of the electromagnetic transformer, piezoelectric ceramic transformers, frequently designated as PT, were developed that make use of the piezoelectric effect. In contrast to electromagnetic transformers, piezoelectric transformers have a precisely limited frequency characteristic of the ratio of the output voltage to the input voltage, where the maximum is at the resonance frequency. This resonance frequency is a function of the material constants and the thickness of the material as well as of the entire transformer, and also of the piezoelectric material and the electrodes used.
Piezoelectric transformers have a number of advantages over the known electromagnetic transformers. In particular, given a comparable transformation ratio, the piezoelectric transformer can be constructed significantly smaller than an electromagnetic transformer. Furthermore, piezoelectric transformers can be constructed to be non-combustible and generate no electromagnetically-induced noise.
The ceramic bodies used for piezoelectric transformers can be realized different forms and shapes, e.g., as rings, flat plates and the like. Different construction forms of piezoelectric transformers are described, for example, in U.S. Pat. Nos. 6,362,559, 6,346,764 or 6,326,718. A known configuration of a piezoelectric transformer comprises a piezoelectric ceramic disk with a circular construction with segmented electrodes arranged on one or both sides of the disk.
U.S. Pat. No. 2,830,274 teaches a piezoelectric transformer of the so-called “Rose type” that exists in various embodiments. The typical Rose type PT comprises a flat ceramic plate that is considerably longer than it is wide and considerably wider than it is thick. The advantages of a piezoelectric transformer in comparison to a magnetic transformer are especially clear here. A piezoelectric transformer can, as already mentioned, be constructed more compactly than a magnetic transformer and has the further advantage of a simpler scalability to lower power.
in the known piezoelectric transformers, a multilayer electrode arrangement with differently segmented primary and secondary electrodes is used, as is schematically shown in FIG. 1.
Such a piezoelectric transformer offers, in particular in connection with switch mode power supplies, an interesting potential for minimizing the dimensions of such a switch mode power supply, as is of great importance especially in connection with portable devices.
For power supplies in the low-power range, i.e., for power below 100 W, in particular below 10 W, primarily switch mode power supplies are used today instead of out-of-date 50 Hz linear power devices. The advantage of the switch mode power supply technology first consists of the reduced construction size and the reduced weight of the apparatus in comparison to a linear device.
In a piezoelectric transformer, the energy is transferred by mechanical oscillation from the primary to the secondary side. Piezoelectric transformers are manufactured from suitable ceramics and have a primary-side and a secondary-side electrode area, as FIG. 1 schematically shows.
The output voltage and the output current of a switch mode power supply are usually regulated independently of the type of transformer, i.e., the output voltage is constant independently of the load to be supplied and up to a maximum current. When the maximum load is exceeded or in the case of a short circuit, the current is maintained constant and thus limited. Most of the regulated switch mode power supplies in the low-power range are regulated on the primary side for reasons of cost, i.e., the regulating apparatus is located on the primary side of the switch mode power supply.
Patent DE 102 59 088 describes such a switch mode power supply with piezoelectric transformer. The output voltage on a variable load is maintained constant here by varying the working frequency and the ratio of cutting-in and cutting-out periods. For this, information regarding the output current is transmitted from the secondary side to the primary side. This transmission must, in order to maintain the pertinent norms, take place in a galvanically separated manner and an optocoupler with appropriate peripheral wiring is usually used.
Patents EP 1146630 and EP 1211794 describe a switch mode power supply with magnetic transformer and regulated on the primary side, in which power supply the output voltage is maintained constant by pulse-width modulation. The information about the level of the output voltage is gained with the aid of an additional winding arranged on the converter transformer. An auxiliary voltage proportional to the output voltage can be tapped off on the auxiliary winding in these known regulating circuits. The auxiliary winding is associated with the primary side of the switch mode power supply and therefore no additional galvanically separated transmission by optocoupler or signal transmission from the secondary side is required. This reduces the number of structural components and results in significant cost savings. However, such known switch mode power supplies controlled on the primary side still contain an electromagnetic transformer with all its above-cited disadvantages.
The present invention is therefore based on the problem of indicating a piezoelectric transformer that can be used in a switch mode power supply controlled on the primary side with the active principles of patents EP 1146630 and EP 1211794. Furthermore, the present invention is based on the problem of indicating an associated control circuit for such a switch mode power supply.